Zinc-plating bath for bright or glossy coating

ABSTRACT

An alkaline cyanide-free zinc electroplating bath. The bath contains sodium hexametaphosphate and such gloss-controlling additives as gelatine, furfural and sulphite liquor from cellulose production.

lJnitefl States Patent Bereczlry et al.

[451 Jan. 25, 1972 ZINC-PLATING BATH FOR BRIGHT 0R GLOSSY COATINGInventors: Tiberiu Bereczky; Ernest Griinwald, both of Cluj; loanCrisan, Bucharest, all of Romania Ministerul llndustriel Constructilorde Masini, Bucharest, Romania Filed: Sept. 25, 1968 Appl. N0.: 762,642

Assignee:

US. Cl. ..204/55 R Int. Cl. ..C23b 5/10, C23b 5/46 Field of Search...204/55, 43, 44, 114, 116;

Primary ExaminerG. L. Kaplan Attorney-Karl F. Ross 57 ABSTRACT Analkaline cyanide-free zinc electroplating bath. The bath contains sodiumhexametaphosphate and such gloss-controlling additives as gelatine,furfural and sulphite liquor from cellulose production.

2 Claims, No Drawings ZINC-PLATING BATH FOR BRIGHT OR GLOSSY COATINGFIELD OF THE INVENTION Our present invention relates to alkalinezinc-plating baths omitting cyanide for producing bright glossy coatingsand, more particularly, to a coating'bath and to a method of platingzinc using this bath.

BACKGROUND OF THE'INVENTION plexed with cyanide ions, in order to formsmooth and glossy surfaces. Besides the zinc ions and the cyanide ionssuch baths contain hydroxyl ions. Such baths may also contain additivesinfluencing the various parameters of the coatings, generally organicand inorganic compounds, capable of modifying the grain size of thedeposited zinc or of promoting a glossy coating. Such additives mayinclude aliphatic aldehydes, aromatic aldehydes or ketones, heterocycliccompounds with oxygen, condensed substances with benzene rings,thioureas, the reaction products of heterocyclic aldehydes with nitrogenor aromatic amines or gelatine. Metallic additives include manganese andmolybdenum directly or in the form of compounds thereof.

In aqueous baths, the additives are not readily soluble and do notdirectly determine a glossy finish on the coating. In fact, the coatinghas a brownish or slightly yellowish coloration which must be removedsubstantially by treatment of the coating with an inoxidizing agent.

Cyanide-contanining alkaline baths also have the disadvantage that theyare toxic and cannot be disposed of without difficulty, that they aredangerous to work with, that they have a poor current efficiency, andoperate with low-current density. To avoid these disadvantages, it hasbeen suggested to provide alkaline zinc-coating baths without cyanideand using other complexing agents. Zincate bathsmay operate atefficiencies close to 100 percent, but produce large-crystal dark andspongy coatings. Pyrophosphate baths require such careful control of theplating parameters that they have been found to be impractical.Moreover, admixture with other ingredients is necessary to obtain glossydeposits. The use of aliphatic amines gives glossy zinc coatings atlow-current densities, but without a desirably large dispersion power sothat with increased current densities brittle coatings are obtained.Much may also be made of the use of sodiumdodecyldiphenyloxidedisulphonate. These baths also have limited valuebecause of the low-current density under whichtheyoperate.

OBJECTS OF THE INVENTION The principal object of our present inventionis to provide an alkaline zinc'plating bath for the production of brightglossy zinc coatings which will obviate the disadvantages mentionedearlier.

It is also an object of this invention to provide a method of platingzinc onto a substrate using an alkaline plating bath which is free fromcyanide and yet provides good dispersion power and is capable oftheefficient deposition of bright zinc coatings of good mechanicalproperties at high rates.

SUMMARY OF THE INVENTION We have now discovered that these objects canbe obtained with an alkaline zinc-plating bath which contains sodiumzincate, omits cyanide and utilizes as the complexing agentsodiumhexametaphosphate. Surprisingly, the complexing agents tie up the zincions as complexes with weak disassociations and thereby reduce theconcentration of uncomplexed zinc in solution. As a result, the cathodicpolarization increases and zinc with a fine microstructure structure isdeposited.

Advantageously, auxiliary gloss-promoting additives are employed inconjunction with the sodium hexametaphosphate. Typical among theseadditives are furfural, gelatine and waste sulphitic lye as resultingfrom the processing of cellulose and utilized in concentrations of L0 to5.0 g./liter. These additives may be used individually or together and,depending upon their relative proportions, produce a gloss ranging intexture from a semigloss to a bright reflective optical gloss. The glossis formed without subsequent treatment.

More specifically, the alkaline zinc-plating bath according to thepresent invention contains 40 to 75 g./liter of sodium hydroxide, 10 to25 g./liter zinc oxide, 30 to 45 g./liter sodium hexametaphosphate andthe above-mentioned gloss-promoting additives. It may be noted,incidentally, that other compounds hitherto utilized for promoting glossmay be introduced in place of the three preferred additives mentionedearlier, or together with them. Such compounds include the organicmembers mentioned earlier and inorganic compounds such as ammoniummolybdate and sodium sulfite.

We may also introduce additives for increasing thepower of dispersion,the adhesion of the deposit to the substrate and the conductivity of theelectrolyte in addition to additives for the prevention of poreformation and wetting agents. The bath may be prepared by combining zincsulfate and sodium hydroxide, thereupon recovering the zinc-hydroxide.The alkaline zincate solution is mixed with the sodium hexametaphosphatesolution whereupon the gelatine solution and furfural solution areadded; the sulphitic lye, neutralized with sodium hydroxide is added asthe last component. The term sulphitic lye as used herein, is intendedto refer to the sulphite liquor obtained in the production of cellulose,any acidic character being removed by the neutralization. The bath canbe utilized immediately and need not be exposed to electrochemicalprocessing or aging. In use, any losses of the above ingredients fromthe bath can be made up by corresponding addition of ingredients theretoand we prefer to continuously or periodically filter and recycle theelectrolyte during plating.

According to another feature of this invention, the plating temperatureis held between 20 and 24 C. so that no heating of the bath isnecessary. The cathodic current density is l to 4.5 A./dm. where noagitation is provided, but up to 6 A./dm. where drum or other rotativeagents are used. For stationary baths, the working voltage is 0.6 to 1.2volts whereas withagitation it is 6 to 8 volts. Best results areobtained when the plating is carried out to deposit 15 to 60microns/hour of a coating. The glossier the coating desired, the lowershould be .the deposition rate. The current efficiency is about percent.andthe dispersive power is about 40 percent. The coatings havesatisfactory uniformity, form well even in interior or hollow portionsof the substrate and upon surfaces with high complexity. and have goodductility and adhesion. The coating may have a brightness ranging fromthat of a plated chromium to a semigloss and-may be applied to polishedor nonpolished surfaces. Suitable substrates are iron, steel andnonferrous metals and alloys, such as copper, aluminum and brass. Thecoatings can be obtained in various hues, e.g., whitish, yellowish andbluish, and can be substituted in many cases for more expensive cadmiumcoatings.

The corrosion resistance of the coating and the retention of the glossmay be improved by treatment of the surfaces with oxidizing compoundssuch as chromic acid and the nitrogen acids, thereby providing apassivating treatment which may be termed chromation or oxidation. Thecorrosion-resistance of the coating is superior to that obtained withcyanide baths and the plating may be carried out with conventionalplating equipment.

SPECIFIC EXAMPLE Automotive-vehicle spark plugs, turned from bars ofnonalloyed steel on automatic lathes, are coated with zinc in an aqueousbath of the following composition:

15 g./liter technical zinc oxide 65 g./liter technical solid sodiumhydroxide 40 g./liter sodiumhexametaphosphate, 68% P 4 cc./litertechnical furfural 4 g./liter food-gelatine sheets 2 cc./liter sulphiticliquor obtained from a cellulose production plant, neutralized with 25percent sodium hydroxide.

To produce 1 liter of the electrolyte, we dissolve, in 300 to 400 cc. ofwater, the sodium hydroxide and then introduce into this solution thezinc oxide with stirring and agitation. In another 300 to 400 cc. of hotwater is dissolved the sodium hexametaphosphate and the two solutionsare then combined. The furfural dissolved in about 50 cc. of water andthe gelatine dissolved in 50 to 60 cc. of water are added successivelyto the basic solution to which is thereafter added the sulfite liquor.

Zinc plating is carried out on the article with a current density of 3A./dm. and a voltage of 1.2 volts with the parts suspended from thecathode bar at a distance of 300 mm. from the anode. The cathode bar isgiven a reciprocal motion at a frequency of 1 Hz., and an amplitude of25 mm. The parts were previously subjected to alkaline electrolyticdegreasing and slight pickling with hydrochloric acid. Coating wascarried out for to 30 minutes whereupon the zinc-coated parts wereremoved and washed with cold water. The zinc-coating has a bright colorsimilar to that of glossy chromium, is duetile, adherent to the basicmetal and has an effective gloss while the uniformity of the deposit issatisfactory. lf lesser quantities of the gloss-promoting additives areused, the coating is a semigloss.

The zinc coating can be passivated by chromation in a bath containing1.5 g./liter sodium-dichromate, 50 g./liter sodium sulphate, 6 g./litersodium chloride and 4 cc./liter concentrated sulphuric acid. Thetreatment was carried out at 20 C. for 12 seconds and a golden hue wasimparted to the coating.

We claim:

1. An alkaline cyanide-free aqueous zinc-electroplating bath containingin addition to zincate ions in an amount equivalent to 10 to 25grams/liter of zinc oxide, 30 to 45 g./liter of sodium hexametaphosphateand a gloss-controlling additive consisting of about 4 grams/liter ofgelatine and about 4 cc./liter of furfural.

2. The bath defined in claim 1 which consists essentially of:

15 g./liter zinc oxide,

65 g./liter sodium hydroxide 40 g./liter sodium hexametaphosphate (68% P0 4 cc./liter furfural 4 g./liter gelatine and 2 cc./litercellulose-production sulphite liquor neutralized with sodium hydroxide.

2. The bath defined in claim 1 which consists essentially of: 15g./liter zinc oxide, 65 g./liter sodium hydroxide 40 g./liter sodiumhexametaphosphate (68% P2O5) 4 cc./liter furfural 4 g./liter gelatineand 2 cc./liter cellulose-production sulphite liquor neutralized withsodium hydroxide.